1. Field of the Invention
The present invention relates to a hydraulic pump for a power steering system in motor vehicles.
In power steering systems for assisting torque generated in manual steering by using a hydraulic fluid as medium, a hydraulic pump installed in the motor vehicles is utilized as power source for supplying the hydraulic fluid to the power steering systems. It is desirable that the power steering systems provide sufficient steering assistance at low vehicle speed or stop, that is, at low rotational speed of an internal combustion engine. Meanwhile, since resistance generated by contact of tires with a road surface is less at high vehicle speed, i.e., at high rotational speed of the engine, than that at low vehicle speed whereby the steering at the high vehicle speed is relatively stable, the power steering systems are not required to provide so great steering assistance at the high vehicle speed. Accordingly, the hydraulic pump increasing its power output as the rotational speed of the engine increases, is unsuitable per se as power source to the power steering systems.
There have been proposed hydraulic pumps with a flow control valve which permits a predetermined amount of hydraulic fluid to be supplied to actuators of the power steering systems for good power steering operation at the idling or low rotational speed of the engine and reduces the predetermined amount of the fluid to an appropriate value for the power steering operation at the high rotational speed of the engine.
2. Description of the Related Art
A hydraulic pump of such a kind is disclosed in Japanese Patent (Utility Model) Application First Publication No. 5-96477. This pump includes a pressure chamber, a drain passage communicable with the pressure chamber, a discharge passage for delivering hydraulic fluid within the pressure chamber to an actuator of a power steering system, a control orifice disposed within the discharge passage, and a first flow control valve for controlling fluid communication between the drain passage and the pressure chamber in response to a difference between a fluid pressure upstream of the control orifice and a fluid pressure downstream thereof. The control orifice includes a main throttle passage and a subsidiary throttle passage arranged in parallel to each other. A second flow control valve is provided for controlling an opening area of the subsidiary throttle passage in response to a difference between fluid pressures within the discharge passage 8 and slots of a rotor which receive slidable vanes. The second flow control valve includes a spool within a spool bore crossing the subsidiary throttle passage and communicating with the slots and the discharge passage, and a spring biasing the spool so as to increase the opening area of the subsidiary throttle passage.
The conventionally known pump allows the fluid within the pressure chamber to be divided into a controlled fluid flow passing through the main and subsidiary throttle passages of the control orifice and an excess fluid flow fed from the pressure chamber to a reservoir tank via the drain passage opened in response to the difference between the fluid pressures upstream and downstream of the control orifice. The controlled fluid flow through the main and subsidiary throttle passages is fed to the actuator to provide the steering assistance required at the low rotational speed of the engine. On the other hand, if the rotational speed of the engine exceeds a predetermined value, then the fluid communication of the drain passage with the pressure chamber increases and the controlled fluid flow is limited to a main fluid flow passing through the main throttle passage by restraining an auxiliary fluid flow through the subsidiary throttle passage. Thus, the fluid flow delivered to the actuator is reduced.
In the conventionally known pump, the spring biasing the spool of the second flow control valve is exposed to the fluid flow passing through the subsidiary throttle passage. It is likely that the fluid flow strikes the spring and causes a so-called Karman vortex to vibrate the spring. This may disturb smooth movement of the spool within the spool bore, resulting in unstable flow control characteristic of the second flow control valve.
It is an object of the present invention to provide a hydraulic pump for power steering systems which is capable of supplying hydraulic fluid having a stable flow characteristic.